Heretofore, as refractory materials in the field of continuous casting, e.g., refractory materials for use in a ladle long nozzle used for the purpose of an oxygen-free process between a ladle and a tundish, an immersion nozzle used for the purpose of control of molten steel fluidity between a tundish and a casting mold, an SN plate used for the purpose of control of molten steel flow rate, an Al2O3—SiO2—C based refractory material and an Al2O3—C based refractory material excellent in thermal shock resistance have been widely employed.
Meanwhile, along with recent diversification of steel grades, the cause and severity of damage to a refractory material used in continuous casting have been increasingly strongly influenced by a component supplied from molten steel. For example, as regards high-Mn steel, Ca-treated steel, high-oxygen steel as typified by porcelain enamel steel, or the like, under continuous collision of molten steel against a refractory material, inclusions existing in the molten steel (in this specification, such inclusions existing in molten steel and consisting of non-metal components will hereinafter be also referred to as “slag”), such as (FeO), (MnO), (CaO) and (V2O5) (in this specification, a chemical component enclosed in parentheses means that it is a component contained in slag) react with the refractory material to produce a highly erosive composite oxide at a contact interface therebetween, and the resulting composite oxide reacts with the refractory material, while penetrating inside a refractory microstructure, to continuously form a low-melting-point substance. In conjunction with a decarburizing action of components of the molten steel on the refractory microstructure, and a washing-down action of stream of the molten steel on the formed low-melting-point substance and others, the low-melting-point substance significantly accelerates damage to the refractory material, thereby becoming a factor for deterioration in durability.
Thus, in the Al2O3—SiO2—C based refractory material commonly applied to continuous casting nozzles, as the most common measure to enhance damage resistance, there have been tried various techniques, such as a technique of reducing a carbon content so as to prevent microstructural degradation due to decarburization, or a technique of reducing or eliminating an amount of SiO2 in the refractory material, which can be formed as a primary component causing lowering of melting point, through reaction with the slag or the like. Although the reduction in SiO2 or C has a certain level of effect, it involves an increase in thermal expansion amount, thereby causing a problem that the risk of crack formation increases due to deterioration in thermal shock resistance. Moreover, the Al2O3 component added as a primary aggregate to the conventional refractory material is formed as a low-melting-point substance through reaction with oxides such as (FeO), (MnO), (CaO) and (V2O5). Therefore, the above techniques fail to obtain a sufficient effect, in fact.
In view of this situation, there have been proposed various refractory compositions obtained by replacing a part or an entirety of the Al2O3 aggregate with an aggregate component which is less likely to react with the above oxides as components of the slag.
For example, the following Patent Document 1 proposes an alumina-magnesia-graphite based refractory material produced using a composition obtained by adding magnesia having a particle size of 0.02 to 1.0 mm or less to a mixture primarily comprised of alumina and graphite, in an amount of 3 to 60 weight % or less, or a refractory material comprising the alumina-magnesia-graphite based refractory material and spinel contained therein.
The Patent Document 2 proposes a continuous casting nozzle having an inner bore portion a part or an entirety of which is constructed of a refractory material comprises spinel and periclase as a mineral phase, wherein an amount of impurities other than Al2O3 and MgO making up spinel and periclase is 3 weight % or less.
The Patent Document 3 proposes an immersion nozzle having a nozzle body constructed of a spinal-periclase-graphite based refractory material comprising spinel: 50 to 95 weight %, periclase: 3 to 20 weight %, and graphite: 5 to 30 weight %, with the remainder being unavoidable impurities: 3 weight % or less.
As in examples of the above Patent Documents, an MgO component such as magnesia (periclase) or spinel has heretofore been selected in many cases, because it is less likely to form a low-melting-point substance through reaction with the slug components such as (FeO), (MnO), (CaO) and (V2O5), as compared to the Al2O3 component.
However, magnesia has a thermal expansion rate greater than that of alumina. Thus, when magnesia is applied to a casting nozzle, it causes an increase in the risk of crack formation, and imposes restrictions on applicable portions and the amount of addition of magnesia. For example, in the Patent Document 3 which discloses the composition comprising spinel: 50 to 95 weight %, periclase: 3 to 20 weight %, and graphite: 5 to 30 weight %, the MgO (periclase) content is about 20 weight % at a maximum, and, if the content exceeds this value, there arises a problem of deterioration in thermal shock resistance, as described in its specification (paragraph [0017]).
As above, a magnesia aggregate-containing refractory material and a low-carbon refractory material exhibit excellent erosion/corrosion resistance. On the other hand, when these refractory materials are applied to a member requiring thermal shock resistance such as a casting nozzle, they cause an increase in the risk of crack formation due to their high expansion property, and thus impose restrictions on the amount of addition of MgO. Thus, the above conventional refractory materials have a problem that, although the MgO component originally owns excellent erosion/corrosion resistance against the slag components, the excellence is not sufficiently utilized because it has to be partially sacrificed for achieving a balance between thermal shock resistance and erosion/corrosion resistance.
Therefore, there has also been tried an approach to satisfying both of thermal shock resistance and erosion/corrosion resistance, based on lowering elastic modulus by a technique of introducing defects or void spaces into a refractory microstructure.
For example, the following Patent Document 4 discloses a method of producing an MgO—C based unburned brick for use in an SN plate and the like, wherein the method comprises: adding magnesia clinker containing MgO in an amount of 95% or more, in an amount of up to 86 weight %; adding stabilized zirconia (YSZ, CSZ) having a stabilization degree of 80 to 100%, in the form of coarse particles and fine particles; adding unstabilized zirconia (0.044 mm or less) in an amount of 3 to 15 weight % in the form of extra-fine particles; adding 3 to 15 weight % of carbon, metal Al, metal Si and a phenol resin; and subjecting the resulting mixture to kneading, shaping, and hardening heat treatment. The Patent Document 4 relates to an invention intended to enhance thermal shock resistance of a refractory material by utilizing a volume change during crystal transformation of the unstabilized zirconia. In other words, this method induces microscopic defects in a refractory microstructure. Thus, there is a limit on improvement of thermal shock resistance
There has been tried another approach to enhancing thermal shock resistance by coating respective peripheries of aggregate particles with pitch or a polymer compound or the like to obtain a raw material; and subjecting the raw material to heat treatment to form a void space around each aggregate particle to thereby lower the elastic modulus of a resulting refractory microstructure.
For example, the following Patent Document 5 discloses a refractory material produced using refractory coarse aggregate particles having an average particle size of 10 to 50 mm, wherein respective surfaces of the refractory coarse aggregate particles are coated with a polymer compound such as phenolic resin, whereby a void space can be formed between a surface of each refractory coarse aggregate particle and a matrix to thereby lower the elastic modulus of the refractory material.
The following Patent Document 6 discloses an MgO—C based unburned brick having a refractory microstructure comprising 10 to 50 volume % of a magnesia particle having a layer formed therearound to have a thickness of 5 to 100 μm and comprised of a void space and pitch. The layer comprised of a void space and pitch can allegedly block propagation of crack to provide enhanced thermal shock resistance.
The following Patent Document 7 discloses a continuous casting nozzle member prepared by subjecting a composition comprising: 80 to 99.5 mass % of a raw material obtained by coating 100 mass parts of a magnesia raw material having a particle size of less than 0.5 mm, with 6 to 30 mass parts of high-softening-point pitch; and 0.5 to 20 mass % of metal powder, to burning in a non-oxidizing atmosphere at a temperature of 500 to 1200° C., wherein the nozzle member has a thermal expansion rate at 1500° C. of 1.5% or less.
Each of the Patent Documents 5 and 6 relates to a technique of preliminarily coating respective surfaces of aggregate particles with a polymer compound, pitch or the like. However, this technique has a problem that a coating agent such as a polymer compound or pitch strongly tends to be unevenly distributed because refractory raw materials have a particle size distribution, particularly, due to a strong cohesive force of extra-fine particles, and thereby it is difficult to uniformly form uniform coatings on respective surface of the particles. Moreover, due to difficulty in control of coating thickness, it is necessary to add the coating agent in an excessive amount. Furthermore, this technique has a problem that, due to damage or peeling of a polymer compound or pitch coating caused by temperature, solvent, inter-particle friction force and others, during a kneading step, it is difficult to sufficiently obtain an expected quality improvement effect, and thereby equality does not become stable.
The Patent Document 7 discloses a technical concept indicating that it is effective to provide a clearance for absorbing thermal expansion, around each magnesia particle. However, as is obvious from the description that it is impossible to perfectly provide an air layer around each magnesia particle (paragraph [0039]), an ideal refractory microstructure could not be obtained. The Patent Document 7 is intended to solve the problem by providing a coating layer comprising a high-softening-point pitch, around each particle, and, during the course of receiving a thermal load, forming a carbide layer (spring-like layer) having a cushioning property or elastic property, from the high-softening-point pitch, as a suitable material having a gas cavity (air layer) around each particle. Because of coating for extra-fine particles having a particle size of less than 0.5 mm, the control of coating thickness of the high-softening-point pitch becomes harder than those in the Patent Documents 5 and 6, so that there is a problem that it is difficult to sufficiently obtain a quality improvement effect, and equality does not become stable.